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PostPosted: Wed Jan 30, 2019 9:50 pm 

Joined: Tue Jan 29, 2019 9:31 pm
Posts: 1
First post - hello! :D

I have a 1992 (I think) Kona Lava Dome, which survived long untended years in the shed, before being reborn last year with nothing much more than a new bottom bracket, rubbers and skewers (and a fair bit of elbow grease too).

Got it to bits again for a service, and I've realised that there's a bit of a mismatch on the rear derailleur which maybe accounts for the occasional bit of chain skip I get when the chains a bit sticky or something's got a little out of line.

It's got a Shimano RD-M500 with an extra long cage, and a cassette of 30-26-23-20-17-15-13 with chainrings of 46-36-24.

The mismatch is that, looking at the Shimano service instructions for the RD-M500 it states that total capacity should be 38 teeth or fewer for the extra long cage, and the above works out at 39.

Could this account for my chain skip? If so, can I increase the chain tension to help? I think there's a spare hole for the derailleur spring to move to, but not sure whether this is the correct tree to bark up.

Cheers, Dan

Oh, also, the barrel adjuster on the RD is completely rusted up (about the only thing on the bike that had in all that time). The original part number is 52A 9801 - anyone know where I'd go for a replacement? Not sure if I can get it out yet, so it might be better to leave in and just rely on the adjustment at the shifter end.

 Post subject: Re:
PostPosted: Wed Jan 30, 2019 9:58 pm 
Retro Guru

Joined: Sat Oct 10, 2009 12:31 pm
Posts: 324
Chain skip is usually due to worn rings or cassette. Shimano spec for RDs is pretty conservative. Many of us ran road mechs on our MTBs in the early 90s* - these are massively undercapacity but no-one with any idea how to use derailleur gears would ever select the largest chainring and the two largest sprockets, so it doesn't matter.

Even the maximum sprocket size can usually be pushed a bit. I have a 9 speed DA short cage mech happily working the 11-30 cassette on my Moulton TSR. The theoretical maximum sprocket for that mech is 27T and I didn't even need a longer B-tension screw.

*still do - lower weight, better ground clearance for twigs and other debris, and quicker shifting

PostPosted: Sat Feb 02, 2019 9:57 am 
Gold Trader / PoTM Winner / RB Rider
Gold Trader / PoTM Winner / RB Rider
User avatar

Joined: Fri Jul 23, 2010 4:13 pm
Posts: 6817
Location: Royal Forest of Dean (Still looking for the Bear !)
Ref your concerns about the barrel adjuster

So long as you get the right thread size of course (of which I have only seen one size in use) you can replace them easily enough
Give it a good soak in penetrating oil and have a go and getting it out ... SweoBbGUQr

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